Call function devices that work for all the telephones on the telephone line, such as some speed dialers and call restrictors, are typically connected serially between the user's telephone and the telephone link to the central telephone office. In the operation of the speed dialer, dual-tone multi-frequency (DTMF) tones generated by the telephone are received by the device. If it is determined that these tones are representative of a speed dialing code, the speed dialer accesses a lookup table using the code as the address. A telephone number stored at the address is then sent over the telephone link, and the telephone is subsequently connected to the link.
The keystrokes necessary to dial a full 11-digit telephone number, for example, are cut to the number of keystrokes necessary to type the code, usually three or four keystrokes. Alternatively, if the DTMF tones generated by the telephone are simply a telephone number that a user wishes to call, the speed dialer forwards the tones onto the telephone link and connects the telephone to the link.
In this way, these devices add a new capability while still allowing the user to dial normally.
The principle downside of serial-connected speed dialers is the difficulty of installation. Serial connection can be made easily for one telephone, but it presents difficulties for many telephones, since there is often no modular jack available that allows a device to be plugged between the central telephone office and all of the phones on a phone line. This installation often requires an experienced technician, which increases costs.
Parallel-connected call restrictors have been proposed. These devices are usually connected in parallel to telephones across the tip and ring lines of the telephone link to block certain dialing codes. The parallel connection requires no special wiring because the parallel-connected device can plug into a modular jack connected to the phone line and work for all phones on the line. If the desired jack is being used by a telephone, a T-adapter can be used or the device can provide this function. Parallel-connected speed dialers have also been proposed. Such a dialer would be plugged into a modular jack to receive a speed dialing code from other telephones on the link. The parallel-connected dialer then generates a particular phone number that this code indicates.